3of4One of the cracked beams at the Transbay Transit Center in San Francsico.Photo: Transbay Joint Powers Authority

4of4Mark Zabaneh, executive director of the Transbay Joint Powers Authority, says something went wrong with the construction inspection process.Photo: Paul Kuroda / Special to The Chronicle

As Transbay Transit Center officials prepare to repair the broken girders that have kept the building closed for 15 weeks, they acknowledged Thursday that their system of inspections during construction failed.

“Obviously, something went wrong with the process for this to happen,” Mark Zabaneh, executive director of the Transbay Joint Powers Authority, which built and operates the public transportation hub, said at the agency’s meeting Thursday.

In September, just six weeks after the center opened, a construction worker discovered cracks in a support beam. A second cracked beam was soon found in the same location, where the center crosses Fremont Street.

Transbay engineers gouged out chunks of metal from the damaged girder for testing. That pointed toward an imperfection in the steel likely related to the cutting of holes in the beam and welding nearby.

Transbay engineers and designers have devised a simple fix for the broken beams that places 2-inch-thick, 14-foot-long steel plates above and below the fractured flange of the girders. They will be connected with 241 bolts.

“The fix is a sandwich,” Zabaneh said.

One will be installed at each of the two cracked girders as well as on two similarly designed girders that cross First Street but have shown no signs of damage.

An independent review committee assembled by the Metropolitan Transportation Commission at the behest of San Francisco Mayor London Breed and Oakland Mayor Libby Schaaf endorsed the repair design. Contractor Webcor is shopping for materials to begin the repair.

It’s unclear how long it will take to get materials and complete the work, which is part of the reason Transbay officials are reluctant to schedule a reopening. But the review committee has also decided to review 15,000 pages of designs, drawings and other construction paperwork that could point toward other areas that should be inspected for potential fractures.

Michael Engelhardt, a University of Texas structural engineering professor and head of the review panel, said in an interview that the team will focus on locations where thick pieces of steel are used, cuts were made in beams to facilitate welding or multiple welds come together — all characteristics of the cracked girders.

“Our concern is: Can lightning strike twice?” Engelhardt said. “It was a unique set of circumstances, but they came together on Fremont Street. Could they also be present somewhere else in Transbay Center? That’s what we need to find out.”

Should multiple inspections be needed, they could slow the reopening of the transit center, said Dennis Turchon, senior construction manager. While Transbay officials are reluctant to even guess when the center might reopen, timelines displayed during Thursday’s meeting indicated that the repair work could be completed by the end of March.

Also during Thursday’s meeting, a member of an earlier independent committee that reviewed design and early construction, said the design of the transit center, including the damaged girders, was sound, that proper construction permits were obtained and the design satisfied or exceeded building codes. A rigorous quality control process was also employed, officials said.

But, Zabaneh said, the process still failed, and Transbay officials need to figure out why.

Michael Cabanatuan has covered all things transportation for the San Francisco Chronicle — from BART strikes, acrobatic bridge construction and dark dirty tunnel excavations to the surging ridership on public transportation and the increasing conflict as cars, bikes and pedestrians struggle to coexist on the streets. He’s ridden high-speed trains in Japan, walked in BART’s Transbay Tube and driven to King City at 55 mph to test fuel efficiency.

He joined The Chronicle as a suburban reporter and deputy bureau chief in Contra Costa County, and has also covered the general assignment beat. In addition to transportation, Michael covers a variety of Bay Area news, including breaking news events. He’s been tear-gassed covering demonstrations in Oakland and exposed to nude protesters in the Castro District. Michael is also a regular contributor to the City Insider column and blog.